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Raising seedlings under a lamp on a windowsill
A February windowsill is bright enough to germinate most seeds and not bright enough to grow them well. The seed comes up, the stem races towards the glass, and by the time the first true leaves open you have a row of pale threads leaning over the edge of the tray. A lamp over the tray is the usual fix. This page is about seedlings from sowing to the point they leave the house. Cuttings, humidity domes and heat mats have their own page: propagation lights and heat mats.
Why seedlings stretch on a sill
Stretching, or etiolation, is the seedling looking for more light. The stem lengthens between the seed leaves and the compost, it stays thin, and it bends towards the brightest direction. On a UK windowsill in late winter that direction is sideways, through the glass, and only for part of the day. The tray behind the first row gets less again.
Warmth makes it worse. A tray above a radiator, or on a heat mat left on after germination, pushes growth faster than the light can support. The result is a tall, soft seedling that flops when you pot it on. Light and warmth need to rise together, and once the seed is up, light is usually the one that is short.
A lamp directly above the tray changes the direction as well as the amount. The seedlings grow up rather than over, and the back row gets a fair share.
How high to hang the lamp
There is no single correct height. It depends on the lamp, the wattage and the spread of its beam. The RHS artificial lighting guide (opened 14 November 2026) gives one home example: T5 high-output fluorescent tubes placed around 60 cm (2 ft) above seedlings. That is a figure for those tubes. An LED bar or a small panel has its own chart in the box or on the listing, and that chart is the place to start. The wider reasoning, and why a maker's distance is indicative rather than a reading from your tray, is in PPFD, DLI and distance.
Then watch the seedlings, because they tell you more than a chart:
- Long, pale stems with a lean: the lamp is too far away, too weak, or on for too short a day.
- Bleached, papery or stunted leaves: the RHS page notes that bleached or stunted seedlings suggest too much light. Raise the lamp a little.
- Short, sturdy stems with green seed leaves: leave it where it is.
Move the lamp in small steps and give the tray a few days to respond. Raise it again as the seedlings grow, so the tips never touch the fitting. A bar on adjustable hangers or chains is easier to lift than a lamp fixed to the underside of a shelf.
Even light matters more on a tray than on a single houseplant. A single bulb in one corner leaves the far corner in a weak edge of the beam, and those seedlings stretch while the middle ones look fine. A bar along the length of the tray, or a small panel sized to it, keeps the row level. Turning the tray a half turn every day or two also helps on a sill, because the window is still lighting one side.
A dark period every night
Seedlings under a lamp still need a night. A common mistake is to leave the light on round the clock in the belief that more hours means faster growth. Plants respond to the length of the dark as well as the length of the day, and a lamp that never switches off is not giving them that.
The RHS page uses around 16 hours a day for seedlings under its T5 example, and says to avoid running a system longer than needed. This site's working range for seedlings is 14–16 hours on, then off until the next morning. The full argument, and how a window changes the hours, is in hours and a dark period.
Put the lamp on a plug-in timer so the night is the same length every night. Switching it by hand works until the first evening you forget, and then the seedlings get a twenty-hour day followed by a short night.
Heat, damp and the windowsill itself
A windowsill swings between warm in afternoon sun and cold at night, especially behind a closed curtain. The curtain traps the tray against cold glass. If you can, pull the tray into the room at night or leave the curtain behind it.
LEDs run cooler than older lamps, but the fitting and its driver still get warm. Keep the lamp off a plastic lid, keep it clear of the curtain, and keep it away from the splash of watering. Water the tray from below, or with a gentle rose, with the lamp switched off, and wipe up anything that reaches the sill.
What it costs
A lamp over a seed tray is a small load for a few weeks. Work out the cost from the watts on the rating label and the hours on the timer, at your own unit rate or the Ofgem cap average this site uses. The arithmetic and worked examples are in grow light running cost. Ignore any "equivalent watts" figure on the box when you do the sum.
When the seedlings are ready to move on
Pot on when the first true leaves have opened and the roots are holding the compost together. If the plants are going outside eventually, they need a gradual introduction to wind, sun and cold first. That stage is in moving plants outdoors in spring.
Questions
Can I leave the grow light on all night for seedlings?
No. Give them a dark period every night. A timer set for around 14–16 hours on, then off, is the pattern used on this site, in line with the RHS example of around 16 hours for seedlings under lights.
My seedlings are tall and floppy. Is it too late?
Not always. Lower the lamp towards the maker's recommended distance or move the tray closer to it, and turn off any heat mat once germination is done. When you pot on, many seedlings can be set a little deeper so the stretched stem is supported. New growth should come in shorter.
Do I need a heat mat as well as a lamp?
Only for seeds that ask for warmth to germinate in a cold room. A mat does not replace light, and a warm, dim tray makes stretching worse. The mat page explains thermostats and sizes in propagation lights and heat mats.